Conventional manufacturing sequences for making hybrid circuits or printed circuit boards generally include a step of etching a layer of copper (or an equivalently useful metal) in order to produce a patterned, electrically conductive layer. In order to assure that the results of such a process are both acceptable and reproducible, the practitioner must continually or periodically determine, inter alia, the etch rate, and must make adjustments to processing conditions when the etch rate is observed to deviate from a predetermined, acceptable range. The procedure for controlling a given etching process is generally specific to the chemistry of that process. However, a widespread method for monitoring the etching rate is to provide a copper layer with a known, uniform, initial thickness, subject the layer to the etchant, and periodically measure the thickness of the layer as the etching process progresses. Although this method has the advantages that it is independent of etchant chemistry and responsive to the mass-transfer conditions of the etching process, it also has certain disadvantages. That is, it requires many thickness measurements in order to provide an acceptable degree of accuracy. Because each measurement involves the use of specialized inspection equipment, a relatively large amount of time is consumed by the measurements. This may lead to bottlenecks, and, by increasing processing time, may also lead to increased production costs.